This invention relates in general to sewing machines and in particular to a new and useful sewing machine and method for approaching a predetermined end point of a sewn seam which is spaced from an edge of the workpiece.
In an older arrangement disclosed in German patent No. 31 50 141 the shortening of the length of the last stitch before the end is achieved in that the forward movement of the feeder is interrupted by the abrupt lowering thereof when the needle has reached the desired end point of the seam. This measure however, cannot be applied to a sewing machine with needle transport in a simple manner because during forward movement the needle is in the workpiece and its movement would have to be interrupted also. But this involves additional difficulties and considerable expense.
There is an older proposal disclosed in German patent application P No. 33 24 715.3 for the execution of one or more stitches with a shortened stitch length before an end point with the aid of a sewing machine with needle transport. Here the setting gearing for adjustment of the feed length of the cloth feeder and of the needle is readjusted just before execution of the last stitch or stitches in such a way that the cloth feeder and needle also execute only the residual stitch length required in each instance for approaching the end point of the seam.
The invention provides a sewing machine which achieves the approach of the end point without shortening the movement of the feed elements in a forward direction. Thereby an additional novel method for approaching the exact end position in sewing machines with needle transport is now indicated.
In accordance with the method of the invention, the end point of a seam which is spaced from an edge of the workpiece is arrived at with the sewing machine which comprises an adjustable needle and a lower transport body sensing the action of the needle and triggering the process for positioning the needle in the end point during the passage through the edge. In accordance with the invention a pulse generator is coupled with the main shaft of the sewing machine so as to deliver counting pulses for a pulse counter and these are transmitted to a micro-computer which controls the action of the feed as a function of the pulses originating from the sensor and from the pulse generator. With the invention, a desired shortening of the length of one or more of the last stitches before the end point of the seam is effected by moving the workpiece back by a difference between the desired stitch length and the stitch length set by the setting device which controls the feed before the execution of the adjustment of the feed. For correction of the stitch length which is carried out during the feed phase the cloth feeder is connected with a switching gear which is controlled by the micro-computer and effects a controlled switching on of the feed in a reverse feeding phase which precedes the respective forward feeding phase. To obtain a stitch length shortened relative to the set stitch length, the lower feed means for feeding the material is connected with a switching gear which operates the feed under the control of the micro-computer during its reverse feeding phase preceding the respective forward phase. A switching gear advantageously comprises an articulated link transmission which is connected at one end with a crank and with a lift drive for the feeder at the other end by means of a lever arm which is mounted within the housing of the sewing machine. With the device and with a method of the invention, it is possible to utilize the backward movement of the cloth feeding mechanism for the correction of the stitch length executed during the forward movement. By using the articulated link transmission of the invention, a sufficiently rigid connection between the lift drive and the cloth feeder is effected for normal sewing.
One link of the articulated link transmission is held in a stretch position by a spring to effect the outward flexing thereof along with a nose member which projects into the path of a control member. This ensures an exact lifting of the lower cloth feeder in its normally inactive phase with the simultaneous outward pivoting of the articulated link transmission.
Accordingly, it is an object of the invention to provide an improved method for approaching a predeterminable end point of a seam which is being sewn.
A further object of the invention is to provide a sewing machine having means for effecting the precise sewing of a seam along an edge point and which is simple in design, rugged in construction and economical to manufacture.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying drawings and descriptive matter in which a preferred embodiment of the invention is illustrated.